Measuring Mass of a White Dwarf

Century-Old Relativity Experiment Used to Measure A White Dwarf’s Mass[1]

This illustration reveals how the gravity of a white dwarf star warps space and bends the light of a distant star behind it. White dwarfs are the burned-out remnants of normal stars. The Hubble Space Telescope captured images of the dead star, called Stein 2051 B, as it passed in front of a background star. During the close alignment, Stein 2051 B deflected the starlight, which appeared offset by about 2 milliarcseconds from its actual position. This deviation is so small that it is equivalent to observing an ant crawl across the surface of a quarter from 1,500 miles away. From this measurement, astronomers calculated that the white dwarf’s mass is roughly 68 percent of the Sun’s mass. Stein 2051 B resides 17 light-years from Earth. The background star is about 5,000 light-years away. The white dwarf is named for its discoverer, Dutch Roman Catholic priest and astronomer Johan Stein.

Astronomers have used the sharp vision of NASA’s Hubble Space Telescope to repeat a century-old test of Einstein’s general theory of relativity. The Hubble team measured the mass of a white dwarf, the burned-out remnant of a normal star, by seeing how much it deflects the light from a background star.

This observation represents the first time Hubble has witnessed this type of effect created by a star. The data provide a solid estimate of the white dwarf’s mass and yield insights into theories of the structure and composition of the burned-out star.

First proposed in 1915, Einstein’s general relativity theory describes how massive objects warp space, which we feel as gravity. The theory was experimentally verified four years later when a team led by British astronomer Sir Arthur Eddington measured how much the Sun’s gravity deflected the image of a background star as its light grazed the sun during a solar eclipse, an effect called gravitational microlensing.

Astronomers can use this effect to see magnified images of distant galaxies or, at closer range, to measure tiny shifts in a star’s apparent position on the sky. Researchers had to wait a century, however, to build telescopes powerful enough to detect this gravitational warping phenomenon caused by a star outside our Solar System. The amount of deflection is so small only the sharpness of Hubble could measure it.

Hubble observed the nearby white dwarf star Stein 2051 B as it passed in front of a background star. During the close alignment, the white dwarf’s gravity bent the light from the distant star, making it appear offset by about 2 milliarcseconds from its actual position. This deviation is so small that it is equivalent to observing an ant crawl across the surface of a quarter from 1,500 miles away.

Using the deflection measurement, the Hubble astronomers calculated that the white dwarf’s mass is roughly 68 percent of the Sun’s mass. This result matches theoretical predictions.

The technique opens a window on a new method to determine a star’s mass. Normally, if a star has a companion, astronomers can determine its mass by measuring the double-star system’s orbital motion. Although Stein 2051 B has a companion, a bright red dwarf, astronomers cannot accurately measure its mass because the stars are too far apart. The stars are at least 5 billion miles apart—almost twice Pluto’s present distance from the Sun.

“This microlensing method is a very independent and direct way to determine the mass of a star,” explained lead researcher Kailash Sahu of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. “It’s like placing the star on a scale: the deflection is analogous to the movement of the needle on the scale.”

Sahu will present his team’s findings on June 7, at the American Astronomical Society meeting in Austin, Texas.

The Hubble analysis also helped the astronomers to independently verify the theory of how a white dwarf’s radius is determined by its mass, an idea first proposed in 1935 by Indian American astronomer Subrahmanyan Chandrasekhar. “Our measurement is a nice confirmation of white dwarf theory, and it even tells us the internal composition of a white dwarf,” said team member Howard Bond of Pennsylvania State University in University Park.

Sahu’s team identified Stein 2051 B and its background star after combing through data of more than 5,000 stars in a catalog of nearby stars that appear to move quickly across the sky. Stars with a higher apparent motion across the sky have a greater chance of passing in front of a distant background star, where the deflection of light can be measured.

After identifying Stein 2051 B and mapping the background star field, the researchers used Hubble’s Wide Field Camera 3 to observe the white dwarf seven different times over a two-year period as it moved past the selected background star.

The Hubble observations were challenging and time-consuming. The research team had to analyze the white dwarf’s velocity and the direction it was moving in order to predict when it would arrive at a position to bend the starlight so the astronomers could observe the phenomenon with Hubble.

The astronomers also had to measure the tiny amount of deflected starlight. “Stein 2051 B appears 400 times brighter than the distant background star,” said team member Jay Anderson of STScI, who led the analysis to precisely measure the positions of stars in the Hubble images. “So measuring the extremely small deflection is like trying to see a firefly move next to a light bulb. The movement of the insect is very small, and the glow of the light bulb makes it difficult to see the insect moving.” In fact, the slight movement is about 1,000 times smaller than the measurement made by Eddington in his 1919 experiment.

Stein 2051 B is named for its discoverer, Dutch Roman Catholic priest and astronomer Johan Stein. It resides 17 light-years from Earth and is estimated to be about 2.7 billion years old. The background star is about 5,000 light-years away.

The researchers plan to use Hubble to conduct a similar microlensing study with Proxima Centauri, our solar system’s closest stellar neighbor.

[1] Source: Space Telescope Science Institute (STScI). “Century-old relativity experiment used to measure a white dwarf’s mass.” ScienceDaily. www.sciencedaily.com/releases/2017/06/170607142604.htm (accessed June 7, 2017).

Dark Energy

Dark Energy[1]

The detection of what is now called “dark energy” is one of the major science discoveries. This discovery is due to the power of the Hubble telescope which gives astronomers their best ever views of the universe.

The gravitational attraction of all the matter in the universe should cause cosmic expansion to slow down. But in 1998, two groups of astronomers discovered the exact opposite: The rate of universal expansion is accelerating. The researchers based their discovery on observations of stellar explosions known a type Ia supernovae, which occur when white dwarf stars grow to their limiting mass of about 1.4 solar masses. Only the Hubble telescope could view the most distant of these explosions and thus confirm the acceleration.

All current studies indicate that a still-mysterious form of energy, dubbed dark energy, propels this speed-up. Although scientists do not yet understand the precise nature of dark energy, they have deduced some of its properties. These efforts suggest that it is the energy associated with empty space, or what scientists call the physical vacuum.

That the vacuum contains energy is not surprising in itself. Quantum mechanics—the physics that describes the universe at the smallest scales—predicts that the physical vacuum is far from empty. Instead, it teems with virtual pairs of particles and antiparticles that appear and disappear within tiny fractions of a second. The problem has been that every theoretical attempt to calculate what the energy density of the vacuum should be has missed the target by several orders of magnitude.

Given the quickening expansion, what will the fate of our universe look like in the distant future? If dark energy does represent the energy of empty space, which as a constant density, then the expansion will continue to accelerate. About a trillion years from now, astronomers living in the merged product of the Milky Way and the Andromeda Galaxy—the two are expected to collide about 4 billion years from now —will not be able to see any other galaxy. The universe then will be well on its way toward a cold death.

[1] See Mario Livio, “Top Seven Science Discoveries,” Astronomy (43, 4, April 2015), pp. 28-35

Anatomy of a Black Hole

Anatomy of a Black Hole[1]

A black hole is a pit in the fabric of spacetime. Space and time, according to Einstein’s theory of special relativity, are interchangeable parts of a thing called space-time: much as width, height, and depth are dimensions of a box, so space and time are dimensions of spacetime. Although the dimensions of space and time are relative and can change, contracting or dilating depending on your frame of reference—an effect noticeable when dealing with strong gravity or relativistic speeds—units of spacetime are absolute.

The figure above gives a cursory explanation and overview of what a black hole actually is. Not only is it a singularity, but it spins. Fast! When astronomers measure a black hole’s spin, they report the value as a fraction of the maximum allowed spin (which would be 1). The bigger member of the black hole binary in the quasar OJ 287 has a spin, labeled OJ 287 has a spin, labeled a, of 0.313, or 31.3% of its max,. What does that mean? This number is related to the angular momentum; it’s not a fraction of the speed of light. But we can turn it into a fraction of the speed of light. That is given (after some messy algebra)

We are pretty sure that black holes really do exist. Stars and gas at the centers of many galaxies orbit around invisible but incredibly massive objects, and we can tell how massive the object is based on these orbits: millions to billions of Suns’ worth of mass. Could it be that we don’t really understand gravity, and something else explains black holes? Yes, but no other ideas have worked out.

[1] Camille M. Carlisle, “Anatomy of a Black Hole,? Sky and Telescope (133, 2, February , 2017), pp. 16-17